This invention relates to speed controlling circuits for controlling the rotational speed of a D.C. electric motor, and is more particularly but not exclusively concerned with much a circuit for controlling the speed of a D.C. motor which is connected to drive a display disc in a flasher type depth sounder.
It is known to control the speed of a D.c. motor at a substantially constant predetermined value by means of a mechanical governor. However, the reliability of such governors is adversely affected by their mechanical construction, and it is difficult to render them adjustable, to permit adjustment of the predetermined constant speed value without substantially increasing their mechanical complexity. This increase in mechanical complexity further reduces the reliability, and increases manufacturing costs.
It is also known to control the speed of a D.C. motor at a substantially constant predetermined value by means of a speed-controlling circuit, which generates a D.C. feedback signal whose D.C. level is proportional to the speed of the motor, and which uses this feedback signal to control the supply of current to the motor. However, such known speed-controlling circuits suffer from the disadvantage that the D.C. level of the feedback signal is sensitive to temperature and supply voltage variations, so that the speed of the motor varies undesirably with changes in ambient temperature and supply voltage. It is difficult to compensate for the effects of these temperatures and supply voltage changes without increasing both the complexity and the cost of the circuits. Moreover, these known circuits normally provide relatively slow starting of the motor from rest.